The present invention relates in general to composite electroless plating, and more particularly, to a process of composite electroless plating without the intentional introduction of certain objectionable materials such as toxic and/or heavy metals, articles resulting from the process, and plating baths used in the process.
The electroless plating of articles or substrates with a composite coating containing finely dispersed particulate matter is well documented.
Electroless plating generally involves the deposition of metal alloys by chemical or electrochemical reduction of aqueous metal ions without introducing an electrical current to activate the ionization process. Through such deposition, the process of electrolessly metallizing a desired metal coating over an article or substrate is achieved.
The fundamentals of composite electroless plating are documented in a text entitled “Electroless Plating Fundamentals and Applications,” edited by G. Mallory and J. B. Hajdu, Chapter 11, published by American Electroplaters and Surface Finishers Society (1990), which is herein incorporated by reference.
As opposed to conventional electroless plating methods, in composite electroless plating, insoluble or sparingly soluble particulate matter is intentionally introduced into a bath solution for subsequent co-deposition onto a substrate or article as a coating.
The evolution of composite electroless plating dates back to U.S. Pat. No. 3,644,183 (Oderkerken), in which a structure of composite electroless plating with finely divided aluminum oxide was interposed between electrodeposited layers to improve the corrosion resistance. Thereafter, U.S. Pat. Nos. 3,617,363 and 3,753,667 (Metzger et al.) extended the Oderkerken work to a great variety of particles and miscellaneous electroless plating baths. Thereafter, Christini et al., in U.S. Pat. No. RE 33,767, further extended the composite electroless plating technique to include the co-deposition of diamond particles. All of the foregoing references are herein incorporated by reference.
The co-deposition of particles in composite electroless plating can dramatically enhance existing characteristics and even add entirely new properties. These capabilities have made composite electroless coatings advantageous for a variety of reasons including, but not limited to, increased utility in conditions requiring less wear and lower friction; facilitating the use of new substrate materials such as titanium, aluminum, lower cost steel alloys, ceramics, and plastics; allowing higher productivity of equipment with greater speeds, less wear, and less maintenance related downtime; and replacing environmentally problematic coatings such as electroplated chromium or nickel. In this last regards, for example, composite electroless coatings with nickel even provide an additional environmental advantage over conventional electroless nickel coatings, which do not include particulate matter, in that the particles within composite electroless nickel coatings reduce the amount of nickel alloy used. Specifically, composite electroless nickel plating reduces the amount of nickel introduced to the environment by a percent equal to the volume percent of the particulate matter within the composite electroless nickel coating.
In addition, composite electroless coatings are regenerative, meaning that their properties are maintained even as portions of the coating are removed during use. This feature results from the uniform manner with which the particles are dispersed throughout the entire plated layer.
However, known composite electroless plating processes suffer from a number of disadvantages. In particular, composite electroless plating baths are inherently unstable and prone to decomposition. To overcome this instability, the standard approach is the use of toxic and/or heavy metals in the plating baths. This incorporation of toxic and/or heavy metals into the plating baths presents multiple challenges. The toxic and/or heavy metals must be added in a sufficient amount to prevent the decomposition of the plating bath, but an increased concentration beyond the necessary level required to prevent the decomposition results in cessation or reduction of the plating rate.
In addition, any materials introduced into the plating bath is likely to be included in the plating. Because regulations directed toward environmental protection preclude disposal of toxic and/or heavy metals, and such regulation has been expanding, a need exists for a plating process which combines the positive features of composite electroless plating without the use of environmentally and regulatory unfriendly materials.
Accordingly, there is still an unsolved need for further improvements in composite electroless plating methods.